• Braga, Daniel
• Moreira, Pedro
• Infante, Virgínia
• Vidal, Catarina

Abstract

<p>Advanced joining processes are key enablers in structural design. In order to overcome the evermore stringent requirements of vehicles and machinery, their structures must be highly optimized, multipurpose and capable of enduring complex load cases and environmental conditions. In this context, structural joining technology is a critical factor for achieving such goals. The limitations imposed by joints may require mitigation through structural design, although in the case of advanced technologies, they may also serve as key enablers, through high strength, low mass, multi-material joints. One set of joining technologies that fits into this category is solid-state joining processes. The solid-state joining nature involves lower heat inputs and as such mitigates or eliminates many of the disadvantages of conventional fusion processes, while at the same time resulting in higher performing joints with the ability of being dissimilar material joints. One solid-state technology that has been the focus of the research community and several industries alike is Friction Stir Welding (FSW). FSW was initially developed at The Welding Institute (TWI) in 1991 and was mainly focus in welding of softer metal alloys, such as aluminum alloys (Thomas et al., 1995). In its most basic configuration, FSW is performed by having a tool composed of a shoulder and probe made of a harden alloy that rotates, plunges, and moves along the abutting faces of two workpieces to be joined. The rotation generates friction heat between the tool and the workpieces, which softens the material to join. The constant relative movement of the tool regarding the workpieces, causes mixing of the two materials to be welded. The relative tool movement may be achieved either by moving the tool along an axis while the workpieces remain stationary, or by having the tool axially stationary while the table containing the workpieces moves. The excellent mechanical properties and the potential of the technique led to extensive development, including the extension to joining other materials and dissimilar materials, as well as new variant processes (Magalhaes et al., 2018). Examples of such variant processes are Friction Stir Processing (FSP) (Mishra et al., 1999), Friction Stir Channelling (FSC) (Vidal et al., 2019; Vidal et al., 2020), Friction Stir Deposition (FSD) (Phillips et al., 2019), Refill Friction Stir Spot Welding (RFSSW) (Brzostek et al., 2018) and Friction Riveting (FricRiveting) (Pina Cipriano et al., 2018), among others. Regarding materials that have been successfully joined through FSW beyond aluminum alloys, much effort has been put onto steel joints (F. C. Liu et al., 2018), but titanium alloys have also been researched (Mironov et al., 2018). These materials present particular challenges, due to the high weld temperatures and the considerable flow stress countering the weld tool as it moves through the workpiece (Reynolds, Tang, Gnaupel-Herold, et al., 2003). In the case of titanium alloys, the low thermal conductivity and reactivity to oxygen creates further challenge to FSW of these alloys (Wu et al., 2014). In order to extend the application cases of FSW, hybridization, primarily with the combination of FSW and adhesive bonding, has been investigated (Braga et al., 2019). By integrating adhesive onto FSW lap joints it is possible to overcome the limitations of these joints, mainly the stress concentration at the weld edges left by the material stirring. These leads to the increase of quasi static strength, fatigue life and corrosion resistance. In this chapter, the principles of friction stir welding will be presented, including metallography, mechanical behavior of joints, fatigue, and fracture. Firstly, welding of aluminum alloys will be addressed as they were the original alloys to be friction stir welded and for which more extensive research was conducted. Following, welding of harder alloys, such as steels and titanium alloys will be addressed. Advances in welding of polymers and composites through FSW and FSW variants will then be discussd. Hybridization of FSW will be discussed focusing primarily on the combination of FSW and adhesive bonding. Dissimilar material joining through FSW will be discussed. Finally, more recent variants of solid-state processes, such as the FSC, a technology of manufacturing, in a single step, continuous sub-surface channels into monolithic metal components and, the FSP, presented as a method of manufacturing multifunctional metal matrix composites will be reviewed and discussed.</p>

Topics

• Deposition
• impedance spectroscopy
• surface
• polymer
• corrosion
• Oxygen
• aluminium
• strength
• steel
• fatigue
• composite
• titanium
• titanium alloy
• thermal conductivity
• joining